The highest day temperature in September 2025 was 88°F. The lowest night temperature was 45°F. The average values for day and night temperatures during September were 75.9°F и 54.3°F respectivly.

Most of the days in September are sunny but you can also expect cloudy and gloomy days.

Precipitation total in September for Boulder is 0.8 in.

The average wind speed in September is 6.5 mph. This value is relatively stable throughout the year.

We are using day length and cloud cover information for each day of the month to calculate averages for this chart. Average sunny hours for Boulder in September is 9.4 hours.